Engine misfire is a fault in the operation of an internal combustion engine, such as typically used in motor vehicles, wherein an air-fuel charge fails to ignite inside the engine cylinder's combustion chamber. In some cases engine misfire may manifest itself as engine stumble, rough idle, and/or vehicle emission control system malfunction. Engine misfire will typically trigger illumination of a malfunction indicator lamp (MIL) on the vehicle's instrument panel, which serves as a warning and a reminder that the vehicle requires servicing and repair.
Correct proportion of fuel and air inside the combustion chamber at time of cylinder firing is required to avoid engine misfire. In a modern internal combustion engine, correct proportion of fuel and air for a specific engine speed and load is determined by precise metering and supply of fuel. Fuel is typically delivered to the combustion chamber by opening an electromechanical fuel injector in response to a control signal of specified duration, i.e. of predetermined pulse-width. Control signal pulse-width is typically programmed to follow a mathematical curve or algorithm determined to provide adequate engine performance. Typically, to achieve a correct proportion of fuel and air at lower engine speeds and loads, particularly at idle, a smaller pulse-width control signal directs an injector to open briefly in order to deliver a smaller amount of fuel.
At times, response of a fuel injector may not properly correspond to the signal pulse-width due to the injector's internal mechanical hysteresis, i.e. friction associated with injector's moving components. During small pulse-width operation, the magnitude of such internal hysteresis may be significant enough that the injector is unresponsive because it remains in a closed position or is slow to open. An unresponsive injector may prevent the proper amount of fuel from being delivered to the combustion chamber, and thus cause the respective cylinder to misfire.